It has recently been suggested that microporous hollow fibers (MHF) may be employed in a liquid membrane separation technique whereby feed and sweep gases flow through the lumens of two different sets of hydrophobic MHF (designated feed-fibers and sweep-fibers, respectively), while a liquid on the shell side of the MHF serves as the membrane. See generally, Majumdar et al, "A New Liquid Membrane Technique for Gas Separation", AICHE Journal, vol. 34, No. 7, pages 1135-1145 (1988), and Sengupta et al., "Separation of solutes from Aqueous solutions by Contained Liquid Membranes", AIChE Journal, vol. 34, no. 10, pages 1698-1708 (1988), the entire content of which is expressly incorporated hereinto by reference. This so-called "contained liquid membrane" (CLM) technique is reported to have several advantages over conventional immobilized liquid membrane (ILM) separation technology.
For example, conventional ILM technology typically requires periodic replacement of the immobilized membrane liquid due to solute saturation, depletion and/or contamination (depending upon the type of separation being conducted). As a result, conventional ILM technology is typically only limited to batch separation processing. However, since the membrane liquid according to the recently proposed CLM technique is physically present in the shell-side of a separation module, it may be replenished and/or replaced more or less continually thereby allowing separation processing to be accomplished on an essentially continuous basis.
Modules for performing CLM separation processes typically include a bundle of microporous hollow fibers divided approximately equally into a set of feed-fibers (through which the feed fluid flows), and a set of strip-fibers (through which the strip fluid flows). The MHF bundle is physically housed within a module case of desired size and configuration such that the lumens of the feed- and strip-fibers are in fluid-communication with supply and discharge ports of the module case associated with the flow of feed and strip fluids, respectively. In this manner, a cocurrent or countercurrent gas flow through the respective sets of feed- and strip-fibers within the module case may be established.
Theoretically, when performing CLM separations, each of the feed-fibers should be in an immediately adjacent non-contacting relationship to a respective one of the strip-fibers so that the distance therebetween is filled with the membrane liquid. According to this ideal configuration, therefore, a theoretical minimum effective membrane thickness (EMT) is established whereby the closest packing of the feed and strip fibers is achieved so that the distance therebetween is minimized. However, conventional module manufacturing techniques fall far short of the theoretical minimum EMT since individual feed-fibers cannot exactly and reliably be interposed with individual strip-fibers. As a result, groupings of feed-fibers will reside in the module adjacent to groupings of strip-fibers thereby significantly increasing the module EMT over the theoretical minimum value.
In addition, conventional module manufacturing techniques invariably cause feed- and strip-fibers to physically contact one another along substantial extents of the fiber length within the module. As briefly noted above, significant surface contact between feed- and strip-fibers is disadvantageous in CLM separations because effectively no membrane is present between those fibers in contact with one another, a condition which again contributes to significant increases of the module EMT over the theoretical value.
It is towards providing solutions to the above problems that the present invention is directed. Broadly, therefore, the present invention is directed to modules containing hollow fiber membranes adapted to being used for contained liquid membrane separations which exhibit effective membrane thicknesses which are closer to the theoretical value than can be obtained using conventional membrane manufacturing techniques.
More specifically, the present invention is directed to modules having a module case and a bundle of hollow fiber membranes therein which are segregated into feed- and strip-fibers. Important to the present invention, the feed- and strip-fibers in the bundle alternate one with the other so that a selected feed-fiber or strip-fiber will respectively be interposed between an adjacent pair of strip-fibers and feed-fibers. As a result, disposition of the feed-fibers and strip-fibers within the module case is significantly closer to an ideal (theoretical) arrangement with the beneficial result being that greater CLM separation efficiencies may be achieved.
The individual feed-fibers and strip-fibers in the microporous hollow fiber bundle are each generally disposed along respective wave-like (e.g., generally sinusoidal) paths within the module case. That is, the individual feed-fibers and strip fibers will converge upon one another at spaced-apart crossing regions, and will arcuately diverge therefrom in opposite directions. Thus, the sets of feed-fibers and strip-fibers each respectively conform generally to sinusoidal paths within the module case, but are physically out-of-phase 180.degree. with one another. In addition, the individual feed-fibers and strip-fibers will alternate with one another as mentioned briefly above so as to minimize (if not prevent entirely) forming clusters of feed-fibers or strip-fibers (which would therefore increase the effective membrane thickness of the module as noted previously).
The modules of this invention are preferably produced using a jig which establishes the generally sinusoidal paths for each of the feed-fibers and strip-fibers. A continuous length of microporous hollow fiber may therefore be unwound from its spool by effecting relative manipulation between the jig and the spool so as to cause the length of microporous hollow fiber to be positioned in the jig along the respective paths of the feed- and strip-fibers (i.e., so that the feed- and strip-fibers assume their generally opposite sinusoidal disposition).
The feed- and strip-fibers are then positionally restrained on the jig so their general sinusoidal disposition is preserved such that they can later be removed from the jig as a coherent bundle. In this connection, the feed- and strip-fibers are most preferably restrained by means of banding the fibers at each of the crossing regions and then removing the thus banded fiber bundle from the jig. Alternately, when polyolefin hollow fiber membranes are employed, the fibers may be subjected to heat-treatment so as to set the fibers in their position on the jig.
Further aspects and advantages of this invention will become more clear after careful consideration is given to the detailed description of he preferred exemplary embodiments thereof which follows.